Low pressure plasma processing is a known technique developed since the early '80 for the cleaning and activation of small components in electronics. Since then, the technique has been evolved continuously and new processes and new applications have been developed.
One of these new processes is the deposition of coatings onto surfaces to add functions to the substrates, such as better wettability, scratch resistance, liquid repellency, and many more. Examples of surfaces which can be coated using plasma coating deposition include polymers, textile and fabrics, metals and alloys, paper, composites, ceramics, as well as specific products made of these materials or a combination thereof.
For example, Yasuda describes the use of hydrocarbons and perfluorocarbons to deposit water repellent coatings by means of plasma processes (Journal of Polymer Science, vol. 15, pp 81-97 and pp 2411-2425 (1977).
EP0049884 describes a process to deposit fluoroalkyl acrylate polymers on a substrate, using low pressure plasma polymerization of precursor monomers.
WO2004067614 describes a method to deposit a liquid repellent coating on an open cell structure, wherein the coating is deposited throughout the whole structure so as to coat not only the outer surface, but the internal surfaces as well.
US2012107901 describes a four-step process to obtain medical devices with better biomolecule adhesion.
As is known from prior art, low pressure plasma processes can be performed in a closed environment under reduced pressure. In their most simplified way, such processes include the following 5 steps:                Evacuation of the chamber to reach low pressure;        Introduction of reaction gas or gases;        Generation of an electromagnetic field inside the plasma chamber to create a beneficial plasma;        Turning off the plasma generation after a sufficient period of time; and        Venting the chamber until atmospheric pressure is reached, after which the treated substrates can be taken out of the chamber.        
Plasma is formed when an electromagnetic field is generated inside the plasma chamber. This is done by application of a power to the electromagnetic field generation device. In a capacitive plasma equipment, electrodes are mounted inside the plasma chamber. Some electrodes are ground, and the power is applied onto the other electrodes, for example radio frequency electrodes. In an inductive plasma equipment, an conductive coil is wound around the plasma chamber, and the power is applied thereon.
Prior art describes two ways of applying power to generate the plasma, which can be used for both capacitive and inductive equipment. A first way is using continuous wave plasma, wherein the power is set at a certain value substantially higher than 0 W, and wherein this constant power value is maintained continuously during the total processing time, e.g. the power is constantly kept at 50 W during a total processing time of 10 minutes.
A second way is using a pulsed plasma mode, wherein the power is applied in a repetitive on-off sequence with short on-times and long off-times, wherein the power is bundled together in higher power peaks (on-time), so that for short on-times the power is substantially higher than 0 W. During the off-time, the power falls back to 0 W which means no power is applied during the off-times. The duration of the on-intervals and off-intervals can be varied to obtain the best process results for a given chemistry and equipment. In general very short on-times, which give sharp power peaks, combined with longer off-times lead to the best results.
In order to obtain more complex functionalities, for example liquid repellent coatings having such a low surface energy that they repel water and/or oil, or for example functionalization processes to impart long-term hydrophilic properties to a substrate, different types of precursors may be used. A lower degree of water and/or oil repellency is obtained with (often gaseous) precursors that have an easy molecular structure. For example, WO2004067614 describes the use of unsaturated and saturated perfluorocarbons, such as C2F6, C3F6, and C3F8. In the case a surface needs to be rendered hydrophilic, shorter term hydrophilicity can be obtained using gaseous precursors or mixtures thereof that have an easy composition, such as oxygen and argon. Other functionalities may also be obtained, increased or decreased, such as oleophobicity, oleophilicity, friction, stiction, cohesion properties, adhesion properties with specific materials, etc.
In those cases where the hydrophilic properties need to be maintained for a long to permanent period of time, more complex precursors (gaseous, liquid or solid) are required. When higher liquid repellency is required, for example better water and/or oil repellency, complex precursor molecules are often used. These molecules consist typically of different functional groups, for example groups for providing good bonding with the substrate and cross-linking, and groups for repellency enhancement. During processing, it is essential that the right functional groups are made reactive. For example, molecules used to impart liquid repellent properties to a substrate should be deposited in a way that keeps the functional group responsible for the repellent properties of the coating as intact as possible in order to obtain the best performing coatings.
It is known from prior art that for complex precursors the average applied power at which the low pressure plasma process takes place has to be low to keep the functional group of the precursor molecule intact, as described in for example “Pulsed Plasma Polymerisation of Perfluorocyclohexane”, by Hynes et al, Macromolecules Vol 29, pp 4220-4225, 1996. The prior art method for generating a plasma as described therein, however, is not always sufficient for the continuous ignition of the plasma, because the requested low average power may be too low to maintain with commercially available generators and may prevent a good and stable ignition of the plasma.
Applicant noticed that both with a continuous wave power at a constant set power and with pulsed power mode, it is not always possible to maintain a stable ignition of the plasma. This can lead to inferior treatment or coating quality.
More in particular, if optimal functionalities are to be ensured using a continuous wave plasma, a very low value of power input is necessary. The applicant has found that such low values lead to unstable plasma ignition, in particular for plasma chambers of smaller volumes, e.g. up to about 500 l.
This problem can also occur in pulsed-plasma processes and apparatuses, as the power input is reduced to zero Watt in these set-ups for a certain off-time, which can lead to unstable plasma ignition.
Further, pulsed plasma processes suffer from the problem of low deposition or treatment rates, which can be less than 50% of the rates of corresponding continuous wave plasma processes, especially for short exposure times. Clearly, for large-scale or high-duty plasma treatment of substrates, it is important to keep the treatment or deposition rates as high as possible and the exposure time as short as possible. Therefore, pulsed plasma processes are usually not preferred for large-scale or high-duty plasma-treatment applications.
Applicant found that the above problems occur especially in smaller systems, e.g. systems with plasma chamber volumes up to 500 l, e.g. used for coating electronic components and/or devices, garments, complex shaped 3D objects, and many more.
European application EP2422887A1 in name of Oticon AS discloses a coating method for coating the surface of a device with a water and oil repellent polymer layer by exposing the surface to a compound comprising 1H, 1H, 2H, 2H-perfluorodecyl acrylate, exposing the surface to a continuous plasma having a plasma power provided by a plasma circuit, and applying a uniform polymer layer with water contact angle of more than 110°. The plasma power is reduced from an initial higher plasma power to a final lower plasma power, during exposure. The final plasma power is less than 35% of the initial higher plasma power. The coating method can be applied to a communication device such as a hearing aid. The method of EP2422887A1 can be deemed superior to plasma polymerization processes which use a pulsed plasma polymerization, because a coating layer can be deposited in a shorter treatment period due to the continuous input of power.
The present inventors have found that the method disclosed in EP2422887A1 does not lead to satisfactory results with respect to the oil- and water-repellency of the obtained coatings when applied in setups which differ from the setup described in the document. More in particular, problems were experienced when the method was applied in plasma chambers which were smaller than the plasma chamber of 100 liter cited in EP2422887A1 and/or if process times of longer than 5 minutes were needed. By applying a power input per liter of chamber volume of between 0.1 W to 0.3 W per liter, e.g. 5 W to 15 W in a chamber of 50 liter, a stable ignition of the plasma could not be obtained in small chambers during the full length of the treatment period.
A straightforward increase in power input, e.g. higher than 0.3 W per liter, could ensure a stable ignition of the plasma. However, by increasing the overall power input, the average power input also needs to be increased which results in surface coatings of inferior water- or oil-repellency, most likely caused by too many 1H, 1H, 2H, 2H-perfluorodecyl acrylate monomers being broken into smaller fragments before polymerizing to the surface.
Monomer comprising smaller fluorocarbon tails than 1H, 1H, 2H, 2H-perfluorodecyl acrylate are less prone to fragmentation. However, it is highly undesirable that the type of monomers used in a plasma polymerization coating process should depend on the chamber volume. Furthermore, in very small chambers, the power which can be applied and for which no fragmentation of the monomer occurs, can be too small to ensure a stable plasma ignition. In case the power input is increased in such very small chambers to ensure a stable plasma, also monomers with smaller tails could fragmentise in the plasma process.
As an alternative or additional solution for the problem of fragmentation, the power input profile as suggested in EP2422887A1 could be altered in that the initial period of high power input can be shortened and/or the final period of low power input can be lengthened, thereby effectively reducing the average power input. However, such a solution again leads to problems with obtaining a stable plasma ignition during the treatment period.
The present invention provides a solution for the problems with the cited prior art discussed above. Thereto, the present invention provides a coating method in which a stable plasma ignition can be ensured even for small plasma chambers, while still keeping the average power input low, thus allowing use of a wide variety of monomers, including the more complex ones containing perfluoroalkyl chains of 4 carbon atoms or more. Moreover, the present invention also provides an improvement in processing time compared to methods using a pulsed plasma.